There have recently been problems such as the lack of lumber resources and the protection of forests, and it is clear that the felling of forests will become increasingly difficult in the future. Accordingly, it is expected that the supply of board materials such as plywood or the like produced using large amounts of raw lumber materials will become uncertain or insufficient, and it is further expected that the price thereof will increase greatly. Accordingly, wood boards which are obtained through the effective use of thin strands of wood or the ligneous fibers of wood strands, which were conventionally viewed as waste material, have attracted attention, and the application of such wood to various uses has been strongly desired.
In wood boards employing thin wooden strands, ligneous fibers, or the like, the thin wooden strands, ligneous fibers, or the like which comprise the structural elements are commonly integrated by means of a binder and are then molded into an integral structure. At this time, as the dimensions of the structural elements become smaller, the wood board which is obtained becomes homogeneous, and the surface becomes smooth; however, the strength and rigidity thereof decline, and the density increases. When, on the other hand, the dimensions of the structural elements become larger, the strength and density of the wood board approach those of natural lumber; however, such wood boards are non-homogeneous, and the unevenness of the surface thereof tends to increase.
A flooring material such as that shown in FIG. 5, in which a decorative single sheet 3 is bonded to one side of plywood 31, is commonly used. The plywood serving as the raw material of this type of flooring material comprises a board in which a plurality of single sheets of veneer or the like are bonded and layered one atop the other so that the grain directions thereof are perpendicular to one another; this plywood possesses sufficient strength and rigidity for use as flooring material, and the surface thereof is smooth.
Accordingly, in order to use the wood board described above in applications as flooring material in place of plywood, it is necessary that the wood board possess strength and rigidity which is equal to or greater than that of plywood, and furthermore, that the surface thereof be smooth.
However, while commonly-known fiber boards using ligneous fibers obtained by the breakdown of wood as structural elements, for example, possess uniform and smooth surfaces, the strength thereof is insufficient for use as flooring material, and when such wood boards are made thicker in order to increase the strength thereof, the density increases, so that the weight thereof increases, and such boards become difficult to handle.
Furthermore, as shown in FIG. 4, in oriented strand boards, in which thin wooden strands 21 are arranged in essentially the same direction and bonded by means of a binder and then molded into an integral form, the strength is improved, but the surface is non-uniform and possesses great unevenness.
Furthermore, in this type of oriented strand board, the strength is improved; however, such boards are deformed greatly in response to changes in humidity, and whereas plywood has a water-absorption thickness expansion coefficient within a range of 3-5%, based on JIS-A 5908 (Japanese Industrial Standard), the value of this expansion coefficient for conventional oriented strand boards is within a range of 10-20%. Accordingly, even if a smooth surface can be obtained by means of abrasion or the like, the individual thin wooden strands comprising such oriented strand boards are subject to expansion and contraction as a result of changes in humidity, and the smoothness of the surface may be thereby lost.
Accordingly, it was difficult to employ such oriented strand boards as surface layer materials. Furthermore, even in flooring material in which a surface decorative layer such as a veneer or the like having a thickness of approximately 0.3 mm is bonded to the surface of such an oriented strand board, as a result of changes in humidity, irregularities appear in the oriented strand board lying beneath the surface of the flooring material, and this was inappropriate for this type of surface decorative material. In particular, under extremely humid environments, deformation resulting from moisture absorption presented a serious problem.
Attempts were made to prevent deformation as a result of changes in humidity by adding wax as a waterproofing agent to the binder when forming the oriented strand boards; however, it was difficult to uniformly disperse wax, which is an organic material, in the aqueous binder commonly employed as the binder for the oriented strand boards, so that the waterproofing agent is ununiformly distributed in the oriented strand boards. As a result deformation, due to changes in thickness under humid circumstance developed in those portions in which there was little waterproofing agent. Thus sufficient effects could not be obtained.